Conventionally, microwave heating apparatuses for burning fine ceramics by the application of microwaves have been in practical use. In the microwave heating apparatuses, in principle, portions of an object to be burned, which is made of a fine ceramic material or the like, are uniformly heated by the application of microwaves, when the object to be burned is homogeneous. In the initial stage of the burning process, however, the temperature of the atmosphere in the heating apparatus is significantly lower than the temperature of the surface of the object to be burned. Therefore, heat is radiated from the surface of the object to be burned and thus a temperature gradient between the surface and the central portion of the object to be burned is produced, so that cracks are easily produced.
Further, heating performed by the application of microwaves has such a characteristic that the dielectric loss increases towards a portion at high temperature in the same material. Specifically, the efficiency of absorption of microwaves is higher in a portion at a higher temperature than in a portion at a lower temperature. Therefore, once a temperature gradient is produced, the difference in microwave absorption efficiency is further increased to cause local heating, thereby further increasing the difference in temperature and promoting the generation of cracks.
To solve the above-described problem, a microwave heating apparatus has been proposed in which an isothermal barrier for making a temperature gradient from the inside to the outside of an object to be burned isothermal distribution is formed in the heating apparatus to permit the generation of a temperature gradient in the object to be burned and to reduce the generation of cracks. As this microwave heating apparatus, an apparatus having the construction shown in FIG. 18 has been proposed (see, for example, Patent Document 1).
As shown in FIG. 18, a microwave heating apparatus 901 includes a cavity 903 in which a microwave space 902 is partitioned and formed, magnetrons 906a to 906d which are connected to the cavity 903 via waveguides 904a to 904d and provided as microwave generation units to radiate microwaves into the cavity 903, microwave agitators 908a and 908b for agitating the microwaves radiated into the cavity 903, a blanket 910 provided in the cavity 903, and an auxiliary blanket 911 surrounding the blanket 910.
In the cavity 903, at least the inner surface reflects the microwaves into the microwave space 902 to prevent the microwaves from leaking out of the cavity 903.
The microwave agitator 908a has agitating blades 914a and 914c disposed in the cavity 903, a drive motor 916a disposed outside the cavity 903, and a rotation-transmitting shaft 918a through which a rotation of the drive motor 916a is transmitted to the agitating blades 914a and 914c. The agitating blades 914a and 914c rotate about the rotation-transmitting shaft 918a to agitate the atmosphere in the cavity 903.
The microwave agitator 908b has agitating blades 914b and 914d disposed in the cavity 903, a drive motor 916b disposed outside the cavity 903, and a rotation-transmitting shaft 918b through which a rotation of the drive motor 916b is transmitted to the agitating blades 914b and 914d. The agitating blades 914b and 914d rotate about the rotation-transmitting shaft 918b to agitate the atmosphere in the cavity 903.
The blanket 910 is formed by partitioning a burning chamber 923 in which objects 921a and 921b to be burned are placed. The blanket 910 has a double-wall structure in which an outer casing wall 925a and an inner casing wall 925b are provided as walls for partitioning and forming the burning chamber 923.
The outer casing wall 925a is made of a material having heat insulating properties while permitting the transmission of microwaves. Specifically, the outer casing wall 925a is made of alumina fibers, foamed alumina or the like. The inner casing wall 925b is made of a dielectric material which self-heats with microwaves from the outside and allows a part of the microwaves to pass through the inner casing wall 925b into the burning chamber 923. For example, as a dielectric material suitable for the inner casing wall 925b, a high-temperature-range exothermic material is used which self-heats at least as much as the objects 921a and 921b to be burned within a high temperature range of about the temperature at which the objects 921a and 921b to be burned are burned.
The auxiliary blanket 911 forms a heat insulating space surrounding the blanket 910 to prevent the generation of a temperature gradient due to the radiation of heat from the blanket 910 into the atmosphere surrounding the blanket 910. The auxiliary blanket 911 is made of a heat insulating material such as alumina fibers or foamed alumina having heat insulating properties while permitting the transmission of microwaves.
Thus, the partition wall of the blanket 910 defining the burning chamber 923 is formed of the inner casing wall 925b self-heating with applied microwaves and permitting a part of the microwaves to pass through the inner casing wall 925b into the burning chamber 923, and the outer casing wall 925a which surrounds the inner casing wall 925b and is made of a heat insulating material. In parallel with the progress of microwave heating on the objects 921a and 921b to be burned, the temperature of the atmosphere in the burning chamber 923 is increased by the self-heating of the inner casing wall 925b, while the radiation of heat from the burning chamber 923 to the outside is prevented by the outer casing wall 925a. 
Accordingly, the atmosphere in the burning chamber 923 is stably maintained at a high temperature according to an increase in the temperature of the objects 921a and 921b to be burned. Thus, the radiation of heat from the surfaces of the objects 921a and 921b to be burned into the atmosphere surrounding the objects 921a and 921b to be burned is reduced. As a result, a temperature gradient is not easily produced between the surfaces and central portions of the objects 921a and 921b to be burned. In consequence, the generation of cracks due to the generation of a temperature gradient is prevented and burning with improved stability can be achieved.
Further, as a configuration for preventing the generation of a temperature gradient, a configuration having a double-isothermal-barrier structure (see, for example, Patent Document 2) has been proposed.